Generation Mechanism of Whistler Waves at Switchback Boundaries in the Young Solar Wind: Particle-in-Cell Model and Parker Solar Probe Measurements

A noteworthy feature of the Parker Solar Probe's (PSP) first encounter with the Sun was the detection of localized magnetic field deflections, and radial velocity spikes called switchbacks (SB) at distances from 25 to 50 solar radii (RS). At 25-35 RS, SBs boundaries are characterized by localized dips of magnetic field magnitude ([1], [2]) often containing bursts of plasma wave activity with frequencies below 0.2 fce -from tens of Hz to a few hundred Hz in the spacecraft frame [1]. Whistlers propagate in both the sunward and anti-sunward directions. However, significant anisotropy of sunward and anti-sunward waves in frequency and amplitude is observed: the sunward whistlers usually have higher amplitudes and are observed at higher frequencies in the plasma frame than the anti-sunward waves. We have used PSP data to identify the parameters of magnetic field and plasma around the dips of magnetic field magnitude related to switchback boundaries to set up the hybrid PIC plasma model to elucidate the wave generation mechanisms. High amplitude, quasi-parallel whistlers propagating toward the Sun are produced by the cyclotron resonance interaction with hot trapped electrons population in the magnetic dip. The wave parameters are in good agreement with parameters of whistlers collocated with magnetic dips at switchback boundaries observed by PSP at 25-35 RS.

[1] O.V. Agapitov, T.D.d. Wit, F.S. Mozer, J.W. Bonnell, J.F. Drake, D. Malaspina, V. Krasnoselskikh, S. Bale, P.L. Whittlesey, A.W. Case, C. Chaston, C. Froment, K. Goetz, K.A. Goodrich, P.R. Harvey, J.C. Kasper, K.E. Korreck, D.E. Larson, R. Livi, R.J. MacDowall, M. Pulupa, C. Revillet, M. Stevens, J.R. Wygant, Sunward-propagating Whistler Waves Collocated with Localized Magnetic Field Holes in the Solar Wind: Parker Solar Probe Observations at 35.7 RS, The Astrophysical Journal, 891 (2020).

[2] W.M. Farrell, R.J. MacDowall, J.R. Gruesbeck, S.D. Bale, J.C. Kasper, Magnetic Field Dropouts at Near-Sun Switchback Boundaries: A Superposed Epoch Analysis, The Astrophysical Journal Supplement Series, 249 (2020).